Widely spread storage batteries include lead-acid batteries and nickel-cadmiumbatteries. With the latest tendency to size and weight reduction of electronic equipment, it has been demanded to develop storage batteries having high energy density. Further, from the standpoint of environmental conservation, electric vehicles or electrical motorbikes causing no environmental pollution have been exploited for practical application, and development of a high performance storage battery for this use is expected. To meet these demands, development of an alkaline storage battery of a new type such as a nickel-zinc battery and a nickel-metal hydride battery as well as improvement of the existing storage batteries in performance properties have been hastened. Such newly developed or improved storage batteries have already been put to practical use in some fields.
Alkaline storage batteries, e.g., a nickel-cadmium battery, a nickel-zinc battery, and a nickel-metal hydride battery, use nickel hydroxide as a main active material of the positive electrode. However, charging of the nickel hydroxide electrode should be carried out with an electricity quantity corresponding to 105 to 150% of the capacity because charging is apt to be accompanied with oxygen evolution as shown by formula (1) below to cause reductions in charging efficiency of active material expressed by utilization and ampere-hour (Ah) efficiency. This means that the conventional nickel hydroxide electrode cannot achieve a prescribed capacity unless it is overcharged. In the state-of-the-art sealed alkaline storage batteries, the sealed system is maintained by setting the capacity of the negative electrode larger than that of the positive electrode so that the positive electrode is always charged first to the full while the oxygen gas evolved from the positive electrode is effectively absorbed on the negative electrode (recombination of oxygen). For example, in a sealed nickel-cadmium battery, oxygen gas is absorbed on the negative electrode through exothermic reaction shown by formula (2). EQU 4OH.sup.-.fwdarw. O.sub.2 +2H.sub.2 O+4e.sup.- ( 1) EQU O.sub.2 +2H.sub.2 O+4e.sup.-.fwdarw. 4OH.sup.- ( 2)
Small-sized alkaline storage batteries whose capacity is less than about 10 Ah are usually assembled by using a metallic case. Charging of these batteries with constant current is controlled by detecting a temperature rise due to heat generation in the overcharge region by means of a thermistor, etc., by detecting a drop of charging voltage due to oxygen gas recombination, or by detecting an internal pressure rise by means of a pressure sensor. Since the rate constant of the oxygen gas recombination reaction is proportional to the partial oxygen pressure, the higher the pressure of the battery, the more advantageous for the gas recombination reaction. In the case of rapid charging, a cylindrical battery which is less liable to deformation or breakage even if the internal pressure increases is preferred to a rectangular battery. Even in using a cylindrical battery, the charging rate is 1 C at the highest. From the viewpoint of energy density, a rectangular battery is preferred to a cylindrical one for space saving, but its charging rate is 0.3 C at the highest due to lower mechanical strength by pressure increase.
On the other hand, large-sized alkaline storage batteries having a capacity of more than about 10 Ah, such as stationary batteries (industrial batteries) and batteries for electric vehicles, have a rectangular shape for space saving. Their pressure tolerance is not more than 4 kg/cm.sup.2 even in using a metallic battery case, making it difficult to obtain a sealed system except for a float type batteries. Accordingly, large-sized alkaline storage batteries are usually used a vent type batteries needing water replenishment. In particular, because batteries which are repeatedly charged and discharged, such as those for electric vehicles, are usually charged at a charging rate of 0.1 C or more, the gas recombination reaction rate is limited, making it very difficult to obtain a sealed system. Nevertheless, there has recently been a keen demand for a large-sized alkaline storage battery having not only an increased energy density but a sealed system which does not need water replenishment. For use in electric vehicles, since gravimetric energy density is regarded more important than volumetric energy density, a plastic case having lower pressure tolerance than a metallic case is generally employed for weight reduction. However, use of a plastic case increases the possibility that a safety valve operates before the gas recombination reaction rate reaches equilibrium state.
There is proposed a method for charging an alkaline storage battery in which an increase in charging voltage based on the polarization of positive electrode is monitored, but the increase in charging voltage is from only 100 to 150 mV at the highest. Further, since the increase in voltage becomes smaller as the temperature increases, the values obtained should be compensated for a temperature change. Furthermore, the reliability is so low that constant voltage charging does not apply to its system by a phenomenon so-called "thermal run-away", which may lead to a failure of a battery. In addition, since an improvement in performance of the gas recombination reaction at the negative electrode is limited, there has been no means but using an extremely low charging current in charging of large-sized sealed batteries.
As previously described, charging of the conventional nickel hydroxide positive electrode is accompanied with oxygen gas evolution before it is fully charged. If the oxygen gas is not completely absorbed on the negative electrode and dissipates outside through a safety valve, the negative electrode is charged by the electricity quantity corresponding to the quantity of oxygen gas dissipated. While cadmium hydroxide remains in the negative electrode as a charge reserve, it is charged and converted to metallic cadmium. In the course of time, chargeable cadmium hydroxide is exhausted, then hydrogen gas evolution from the negative electrode commences. As a result, the amount of electrolyte decreases, and the capacity of the positive limiting battery on discharge decreases.
The nickel hydroxide electrode has an additional disadvantage of considerable reduction in charging efficiency in high temperatures. This is because a difference between potential of oxidation of nickel hydroxide to nickel oxyhydroxide on charge and that of oxygen evolution becomes small in a high temperature so that a competitive oxygen evolution reaction, tends to predominate in the course of charging. As an approach to an improvement in high temperature performance of the nickel hydroxide electrode, various additives have been studied. Among them, it has been proposed to co-precipitate cobalt hydroxide together with nickel hydroxide to form a solid solution as reported, e.g., in Journal of the Electrochemical Society Japan, Vol. 31, No. 1-2, p. 47 (1963), U.S. Pat. Nos. 3,951,686 and 4,603,094, GS News Technical Report, Vol. 36, No. 2, p. 31 (1977), Japanese Patent Publication No. Sho. 56-36796, and Japanese Patent Unexamined Publication No. Sho. 50-132441. Cobalt hydroxide is capable of forming a solid solution with nickel hydroxide, and there is a tendency that the charging potential becomes less noble according as the amount of the solid solution formed increases. In other words, the difference between the potential of charging and that of oxygen evolution becomes large to suppress the competing oxygen evolution reaction, resulting in an improvement in charging efficiency. The amount of cobalt hydroxide to be added varies depending on the use of a battery. For example, it is from 1 to 8% by weight for batteries subject to cycles of charge and discharge. For trickle use in high temperature, for example, as a power source of an emergency lamp where the temperature reaches 45.degree. C., cobalt hydroxide is usually added in an amount of from 8 to 20% by weight while using a sodium hydroxide solution as an electrolyte. However, an increase in amount of cobalt hydroxide added causes not only a reduction in discharging voltage, resulting in a reduction in energy density, but also an increase in cost. The power source of electric vehicles or motorbikes is used in a wide temperature range of from -10.degree. C. in winter to 60.degree. to 70.degree. C. in summer and should be subjected to repetition of charge-discharge cycles. In seeking for batteries having a higher energy density than lead-acid batteries for this use, alkaline storage batteries using nickel hydroxide electrode, e.g., a nickel-cadmium battery, a nickel-zinc battery, and a nickel-metal hydride battery, are considered as candidates. The substrate supporting an active material of the positive electrode of these alkaline storage batteries include nickel porous substrates, e.g., a sintered nickel substrate, a foamed nickel substrate, and a fibrous nickel substrate. An increased energy density can be obtained by increasing the porosity of these substrates to which an active material is applied. It turned out, however, that a substrate having a porosity of 86% or more filled with an active material undergoes expansion in dimension ascribed to volumetric change of the active material upon charging particularly in a low temperature of 0.degree. C. or below, causing reduction of battery life and that the active material of the positive electrode has a reduced charging efficiency when charged in a high temperature of 60.degree. to 70.degree. C., resulting in a great reduction in battery capacity. These problems particularly arising with batteries using a low pressure-resistant plastic case, it has been impossible to apply to such batteries the accepted know-how relating to addition of cobalt hydroxide, for example, an optimum amount to be added and an optimum composition of an electrolyte.